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73-87 Chevy _ GMC Trucks => Brakes, Frame, Steering & Suspension => Brakes and Braking Systems => Topic started by: monte85 on February 13, 2008, 11:21:29 am
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well heres the problem and i don't know if you other guys out there have had similar problems or not. i have an 87 v20 with the jb7 brakes( over 8500 gvwr). i have the proportioning valve above my rear axle. anyways, when it rains, the brakes will lock up everytime you push the pedal. also the pedal gets real sensitive(alittle pressure will put you through the windshield). this has occured since i bought the truck. i asked my uncle who has dealt with gm stuff for most of his life about it and he said it was common for those trucks(whether he meant the larger 3/4 and 1 tons or all from this vintage i don't know). before i start getting responses, let me say that it does not do this at any other time, just when it rains or there has been alot of moisture in the air. everything in the braking system is also new with the exeption of the master cylinder. i put in all new lines front to back, wheel cylinders, calipers, and hoses. and bled the crap out of everything. i don't mind it really, i was just wondering if there is a way to get rid of it. i never seem to see any of the newbies asking about it either. anyway, if someone out there can shed some light on this, i would appreciate it.
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The rear piece only adds pressure when there is a load on the truck that is what the lever is for.
As for it locking the tires in the rain, how hard are you laying into it? It will DEFINITELY be different than a ABS equipped truck and they are light on the rear when you lay into them. Are you only complaining about it during rain?
How tight are the rear shoes adjusted?
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i never expected it to feel like abs. i've never owned an abs equipped vehilcle. i'm not putting alot of pressure on the pedal either. only enough to slow the truck down under normal conditions(like if you came up on a car traveling alittle slower than you were), but they lock up during wet conditions. i guess i should be more clear about my feelings here. i'm not complaining about it(although it does kinda sound like it). if its the way it is then its the way it is. i've lived with it for almost 4 years now and i just know that the truck will do it. i was just wondering if anyone else has had the same experience. up here in connecticut its raining like crazy now and its done it to me since the rain started. the rears aren't tight. when you spin the tires you might here them rub in a spot or two as the drum spins.
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Well what i do when it rains or i go thur a puddle i ride my brake a lil at low speeds to heat them up...And When stoping in wet condtions its always best to lightly pump ur brakes....insted of using steady pressure try to use 3 light pressure pulses insted of just one long one.....But this advice should only be taken if ur sure u have no leaks and nothing is broke...
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Redneck - if i pumped them 3 times this is what would happen. i would hit my head on the windhield 3 times.
heres an example: in the parking lot at work here, there is space in front of the buildings and that meets up to a main driveway. i'll back out of a space and drive to the main driveway. only going about 5 or 10 mph for less than a minute. i'll apply light easy pedal pressure to slow down for the main driveway. this is enough to make the wheels lock. when they lock, i don't go skidding off sideways, its nice and straight. like if both sides lock.
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Well what i do when it rains or i go thur a puddle i ride my brake a lil at low speeds to heat them up...And When stoping in wet condtions its always best to lightly pump ur brakes....insted of using steady pressure try to use 3 light pressure pulses insted of just one long one.....But this advice should only be taken if ur sure u have no leaks and nothing is broke...
Spoken like a true truck driver. lol.
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man that sounds crazy...... u might wanna let some slack out of them read adujsters and make sure ur wheel cylinder isnt leaking......cause from what ur saying it sounds like the back is locking up way before the front do....
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That is WAY to touchy, the rear prop may be bad, but with only 10mph and light pressure the rears shouldn't lock up.
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well i don't actually know if it is the rears that are locking or fronts. i just know when it happens i hear tires skidding on pavement. and both wheel cylinders are new. where would you even get one of those rear proportioning valves anyway? and if they were adjusted to tight, why would it only do it when it rained?
and i'm guessing from all this talk that this was not a common quirk with these trucks.
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1st thing you need to do is pull your drums. Inspect to make sure you have a primary and secondary shoe on each side and that they are installed correctly. The front shoe will have less meat on it than the rear shoe. There should be no axle grease or brake fluid on the linings. Make sure the shoes are sitting flush on the anchor pin. If they are not someone adjusted the parking brake cable too tight or the parking brake is hanging up. Now once all of that is ok adjust the brakes so there is slight drag on them. Then see what happens, if they still lock, disconnect the arm on the rear proportioning valve and manually move it to see if it makes a difference. If the truck has been lifted you will need to correct for it. If it doesn't make it better then just bypass it.
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well i don't actually know if it is the rears that are locking or fronts. i just know when it happens i hear tires skidding on pavement. and both wheel cylinders are new. where would you even get one of those rear proportioning valves anyway? and if they were adjusted to tight, why would it only do it when it rained?
and i'm guessing from all this talk that this was not a common quirk with these trucks.
Chances are your fronts are locking up and dragging, by the time the rears come to a stop... My truck does this too on slightly damp pavement, otherwise hope you have your seatbelt on when riding with me, cause it stops on a dime and will make change ;D being that you are a Large Pickup, Honestly you should be giving yourself plenty of stopping distance between you and whatever is in front of you, reguardless of your brakes adjusted properly or not....
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I get evil thought about what his truck is doing .wouldnt mind getting my father in law to go on a ride with me....Right when he starts running his mouth "pow" lol "sorry brakes are wet"
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I get evil thought about what his truck is doing .wouldnt mind getting my father in law to go on a ride with me....Right when he starts running his mouth "pow" lol "sorry brakes are wet"
Kinda oops, oh darn, he should have had his seat belt on. Yeah I can here it know honest honey it was an accident.lol.
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You guys should know that most brake shoes do this, no matter make or model, and especially if the shoes have ANY organic material in them. It is referred to as "swelling". Humidity in the air causes the shoes to swell and until they are heated up and the moisture cooked out of them, then you will likely kiss the windshield. I learned this in my 85 celebrity the hard way with no seat belt on just moving the car around in the yard.
Added-> You may want to look at replacing the shoes with some Raybestos or equivelant as they don't contain very much organic material at all.
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i tried the thing that Redneck says he does. i rode the brakes a couple of seconds when i first started driving and it seems to stop them from locking up. thanks for all the help guys.
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You shouldn't have to do that. You need to inspect your brakes
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Monte85, I read your thread last night and was going to comment because that happens to me at times but couldn't remember the circumstances when it happened. So this morning I'm driving my truck (86 K-30) for the first time since the rain the other day and hit my brakes for the first time and almost hit the dash! Rears locked up, luckily it was a light foot. Warmed up the brakes a little the next couple stops and everything was normal. I have the hydro boost with a fresh pump and braided flex lines, everything else is new. The big drums just grab hard when wet and cold, my dump truck would get touchy when wet too.
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That usually happens because of one of the reasons I mentioned earlier. One of the most overlooked problems with duo-servo brakes is what I stated with the shoes not seated on the anchor pin, either by an over adjusted parking brake cable or sticking brake cable. If everything else is ok and they're adjusted properly and they are grabbing that hard that they lock until they've been applied a few times your metering valve is bad.
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yeah, now that the rain has stopped, my brakes aren't doing it. i'll still give them a once over this weekend, but i don't expect to find anything wrong. its done it for 4 years now and i've never found any leaks or blown out wheel cylinders or calipers. i just got used to being careful when putting on the brakes after a good rain.
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My parking brake cables are actually hanging up a little (something I noticed after all the work I just did) and I have a habit of running my rear brakes tight, I'm hauling or towing heavy loads often and like my brakes there all the time. I did eliminate the load sensing prop valve as well when I did the lift.
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It's not the rain getting on your brake shoes guys it's the loss of tractive load and the brakes grabbing. So make sure you check all those key points I mentioned. I can't tell you how many of the exact same complaints I've fixed with these trucks over the years including all of my own that did the same thing.